Sometimes just playing with the Levels tool will not give you enough control over the exposure of your image. However, the Histogram Adjustment tool can easily alter both the exposure on an image-wide scale and in each individual channel with the same tool, seeing your results in real time. This is much easier and once you understand what you are doing with this tool it can save you a lot of time correcting images that may have a problem with their colour or exposure.
In this tutorial I'm going to explain the main features of the Histogram Adjustment tool and show how it can be used to improve an image. The same process applies to fixing an image's exposure too and as such you can use this information to do that too.
Step 1. Open your image in Paint Shop Pro and right-click the Background layer before selecting Duplicate. Double-click the name of the new layer (Raster 1 usually) and type in a new name so that you know exactly what this layer contains; here it will be a version of the image that has been edited with the Histogram tool.
Step 2. Now select Adjust>Brightness and Contrast, Histogram, Adjustment. You can also press Control, Shift and H to do this. A new window will open up containing two thumbnail images that are entirely too small to be of any use and a variety of buttons and a graph. Check the box in the top-right corner to make it so that the result is previewed in your workspace rather than in the thumbnail images.
Step 3. First let's get to understand what exactly the Histogram shows us. The grey graph is the image's original colour information, and the coloured translucent graph is the histogram we will make the image adhere to if we apply the tool with these settings.
If we think about every colour as a mixture of three colour exposures (blue, red and green) with varying shades of grey mixed in to create light and dark we can begin to understand the graph. As you progress along the horizontal axis we are moving from 0 (black) to 255 (white) in these toners, and the height of any point on the histogram shows how many pixels in your image are currently using that tone. This applies to both the compiled histogram (seen using the Luminance option) and the individual channel versions.
If you position your cursor over any point in the graph you can see which shade any point represents from the Input value, the count lets you know how many pixels are that colour and the Output will tell you what value it will appear if you current changes are applied.
Step 4. On the left side there are the Maximum and Minimum drop-down sliders; these govern what value out of the 255 grey tones applied to the colours is the maximum and minimum brightness. So if we set the Maximum setting to half we will have all the brighter shades forced to a mid-grey (The image will now treat the grey tone at 122 as white) and everything that exists between white and black will be forced into one of the shades between the two new parameters. This will darken your image considerably, just as bringing the minimum up would brighten your image although this approach is definitely not the best way to change the brightness of your image since you lose a lot of the tone information in the process.
Step 5. Now we will look at the compression settings. The slider on the right has two small images at each end to demonstrate the shape of the curve that this tool will create; sliding the bit towards the top creates a curve with a very long, flat mid-section, and moving the slider down will cause the curve to transition very abruptly from low values to higher ones by creating a more vertical middle. What this means for the image is that you will either increase the amount of the image that will even out the number of pixels that are each brightness by straightening the middle, reducing contrast as a result of straightening out the middle area of the line, or you can increase contrast by making it so that the mid tones are virtually non-existent by forcing pixels to either be bright or dark. The top half of this screenshot shows you the histogram and resulting image from fully compressing the image into the mid-tones, and the bottom area shows what happens if you expand the mid-tones creating higher contrast.
Step 6. The lowest row of buttons that show an upwards and downwards pointing arrows and a 1:1 control the scale of your histogram and usually can be ignored (the 1:1 returns the graph to its default size so that the highest point of the graph is at the very top). The arrowheads directly underneath the histogram pointing to points on the X axis can also be controlled more accurately using the numbers in the boxes underneath them and the percentage sliders, though generally using the arrows themselves will suffice. These work just like the tags in the Levels tool; there is an arrow pointing at the point that tones will display as black on the left, an arrow at the mid-point controlling the gamma (how bright the tone exactly in the middle will appear) and an arrow on the far right controlling the point colours appear their brightest. In my histogram there is an empty portion to the right of the graph's end; this means that not all of the available tones are being used, so we can left-click the white arrow and drag it to the left so that the red histogram will go all the way across the axis and the arrow is pointing at the end of the original graph. This is working just like the Levels tool since I am in Luminance mode, meaning that I am controlling the exposure of the average output of the three colours. While this works, we can increase our options by not using this method and venturing into the individual colours to correct the exposure.
Step 7. At the top where it says Edit we have two options. The Luminance mode will alter the overall exposure of the image once all three colours have been averaged and combined, meaning that any changes we make to this graph will be made to all three exposures regardless of whether this means the loss of some tone information in one channel or creating an empty space in the histogram of another. This works as a quick fix, but to get the best possible results it is best to select ‘Colours' here and then go through each colour individually. So set the editing mode to Colours and you should be presented with a new histogram representing the tones of the Red channel on its own before it is compiled. Notice that any changes you have started to make in Luminance mode are cancelled when you switch to Colour mode, though they will be reapplied if you decide to switch back to the prior mode of editing.
Step 8. Once again there is an empty area to the right of the histogram for my red tones, since this was present in the Luminance it is almost certain it will be here in all three colour channels, the reason we are altering the image at this level is because some of the colour channels will have different sized gaps in them requiring different corrections. As before, drag the white arrow across to make the graph fill the full range of tones along the X axis, then use the drop-down menu next to the Editing Mode selection to change to the green and blue channels, correcting them as before. This means that you can correct each tone so that your image is better balanced than if you had just used the Luminance mode. The change in my image is not drastic, but it is brighter and we can rest assured that every tone of every colour is now potentially present and that we haven't lost any colour information as a result of quick editing techniques. This method means that you can correct any issues with your exposure whether it's merely an issue of brightness and contrast, or correcting the colour balance in an image.
The Histogram tool is very useful for fixing your images’ exposures and all the major graphics packages have it in some form so have a look around the Colour and Exposure controls in whatever program you use and you’ll find it easily enough. Although it seems daunting at first it soon becomes second nature, and if you think about it as a series of Levels tools you can’t go far wrong. Not only does the histogram tool let you control the exposure of you image, but it
also allows you to correct colour bias that may occur in a photograph and as such it is an important tool to have knowledge of, so putting in a few minutes learning the basics is definitely time well spent.